Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. In one conventional approach, semiconductor wafers are mounted near the periphery of a disk. The disk is rotated about its central axis and is translated with respect to the ion beam to distribute the ion beam over the semiconductor wafers. The ion implanter typically includes an end station having automated wafer handling equipment for introducing wafers into the ion implanter and for removing wafers after implantation.
The wafer handling system typically transfers wafers from a cassette holder to a process station, such as a wafer mounting site on the disk. One requirement is to accurately position the wafer at the process station with its flat or notch having a predetermined orientation. The slots in the cassette holder are somewhat larger than the wafer and thus do not ensure accurate wafer positioning. Furthermore, the wafer flat or notch orientation is not controlled in the cassette holder. However, accurate positioning at the process station is necessary to ensure reliable wafer retention and to avoid wafer damage. In addition, ion implantation systems typically require a particular wafer flat or notch orientation, which is indicative of the crystal orientation of the wafer, to control channeling by implanted ions.
A wafer transfer system incorporating a wafer orienter is disclosed in U.S. Pat. No. 4,836,733, issued Jun. 6, 1989 to Hertel et al. A wafer is placed on an orienter chuck and is rotated. An orientation sensor includes a light source positioned below the edge of the wafer and a solar cell positioned above the edge of the wafer in alignment with the light source. The light beam from the source is directed perpendicular to the wafer surface. The wafer blocks a portion of the light beam from reaching the solar cell. The signal output from the solar cell is indicative of wafer eccentricity and a fiducial, such as a flat or a notch. Based on the signal from the orientation sensor, eccentricity and rotational orientation may be corrected. Wafer aligners are also disclosed in U.S. Pat. Nos. 5,452,521, issued Sep. 26, 1995 to Niewmierzycki; 5,238,354, issued Aug. 24, 1993 to Volovich; and 4,345,836, issued Aug. 24, 1982 to Phillips.
Prior art wafer orientation sensors provide generally satisfactory results with conventional silicon wafers. However, in some instances, the ion implanter is required to operate with wafers of different materials, including but not limited to quartz, sapphire and glass, with either a notch or a flat as the fiducial. For example, quartz wafers may be utilized for testing uniformity and dose in the ion implanter. The conventional optical orientation sensor is unable to sense the edge of a quartz wafer, because the light beam from the light source is not blocked by the transparent quartz wafer, and the wafer is, to a large degree, invisible to the sensor.
Accordingly, there is a need for improved wafer orientation sensors which can sense the edges of wafers of different materials, including transparent materials.